FIG. 1 illustrates a conceptual diagram of a prior art geared rotary actuator which has been manufactured by the assignee of the present invention. The geared rotary actuator 10 functions to transmit input torque applied to input shaft 12 to an output actuator 14 of the type which has been used to activate the control surfaces of an airplane. In order to maintain precise control of the aircraft in flight, it is necessary that the amount of backlash between the input shaft 12 and the actuator 14 be minimized. Torque from the shaft 12 is transmitted to sun gear 16. The shaft 12 is supported by a pair of bearings 18. A plurality of planetary gear sets 20 are radially disposed with respect to the input shaft 12 and evenly circumferentially spaced around the sun gear 16. Each of the planetary gear sets 20 includes a support shaft 22 to which are mounted first, second and third planetary gears 24, 26 and 28, respectively. While the gears 24, 26 and 28 have been illustrated as separate parts, it should be understood that the shaft 22 is typically integral with the aforementioned gears. The first and second planetary gears 24 and 28 respectively mesh with fixed first and second ring gears 30 and 32, respectively which are rotatably supported by bearings 37. The third planetary gear 26 meshes with a third ring gear 34 which is rotatably supported by bearings 36.
First and second annular support rings 36 and 38 rotatably support the support shafts 22 of the planetary gear sets 20. The first and second support rings are solid. Each of the support rings 36 and 38 has an annulus which has an inner diameter substantially larger than the input shaft 12 so that the support rings only function to provide radial spacing of the planetary gears and do not transfer load to shaft 12. Outer surface 40 of the first and second support rings 36 and 38 rotatably supports the outer surface 42 of the support shafts 22.
The actuator 14 is caused to rotate with respect to the fixed ring gears 30 and 32 as a consequence of the ring gear 34 having a different pitch diameter than the pitch diameter of the ring gears 30 and 32. Each of the planetary gears 24, 26 and 28 has a pitch diameter designed to mate with each respective ring gear which causes the aforementioned motion of actuator 14 which, as described above, has been used to power the movement of control surfaces of an airplane.
The prior art actuator described above has deficiencies which are caused by the solid support rings 36 and 38. In the geared rotary actuator described above, the planetary gears 24, 26 and 28 are supported by the support rings 36 and 38 by means of the support rings rotatably supporting the support shafts 22 to which the planetary gears are attached. When a torque is applied to the planetary gears, a load is developed at the planetary gear teeth having both radial and tangential components. The radial load is resisted by the support rings which hold the planetary gears in mesh with the ring gears. The solid support rings are relatively stiff and do not substantially deflect inward in response to radial loads. Depending upon the torsional free play requirement of the actuator 14, the gears may be assembled with very little backlash at the planetary to ring gear teeth meshes. This backlash is controlled by the outside diameter of the support rings 36 and 38. The larger the support ring, the smaller the backlash and torsional free play. However, if the backlash between the teeth of the ring gears and the planetary gears is less than the clearance between the second ring gear 34 and the bearing 37, the planetary gears will carry the shear load. It is preferable that the radial load be transferred to the bearings 37 than to the teeth of the planetary gears 24, 26 and 28. As a consequence of the assemblage of the aforementioned rotary actuator with a small backlash, high shear loads applied to the teeth of the planetary gears 24, 26 and 28 have caused damage to the teeth of the planetary gears which increases the backlash between the input shaft 12 and the rotary actuator 14 which lessens precise control of flight surfaces. Any increase in backlash consequent from damage to the teeth of the planetary gears results in lessening of the ability to precisely control the positioning of the control surfaces of the airplane.
U.S. Pat. No. 4,384,498 discloses a deformable inner ring gear which meshes with a plurality of radially inwardly disposed planetary gears for the purpose of load balance consequent from deviations caused by manufacturing or loads applied to an external housing.
U.S. Pat. No. 4,674,351, which is assigned to the assignee of the present invention, discloses a compliant gear. A compliant laminate is disposed between a hub portion and a rim portion of the gear with the laminate including a rigid laminar shim sandwiched between a pair of elastomer layers.